1. Field of the Invention
The present invention relates to a novel device for estimating central venous pressure.
2. Description of the Related Art
An accurate, quick, and non-invasive means for estimating central venous pressure (hereinafter CVP) is a valuable diagnostic tool for patients suffering from cardiac disease. In a patient suffering from cardiac disease the heart fails to pump sufficient volume through the circulatory system, and this causes blood to "back up" in the veins returning blood to the heart. Excess blood volume which is not pumped through by the heart muscle causes blood pressure in the veins, or venous pressure, to increase which results in distension or expansion of the patient's veins.
A precise measurement of CVP, or the blood pressure in the right atrium of the heart, is often necessary for the diagnosis of a patient with cardiac disease. The most reliable and accurate method of measurement involves the insertion of a catheter into one of the major veins in the circulatory system. The catheter is threaded through the vein until it reaches a position near the heart where a pressure measurement is taken. Such a procedure is invasive, medically risky, and highly traumatic for the patient.
Invasive measures which establish CVP are used only when an accurate measurement is absolutely necessary. Non-invasive methods are used when an estimate of CVP is sufficient for diagnosis. A simple method for estimating CVP is described in B. Bates, L. Bickley & R. Hoekelman, A Guide to Physical Examination and History Taking 270-271 (1995) and in J. Constant, Bedside Cardiology 80-86 (1985). The cited pages are incorporated herein by way of reference.
The physiological basis for the method is that in a healthy individual who is standing or sitting the blood pressure in the internal jugular vein, which is located behind the sternomastoid muscle in the neck, is lower than atmospheric pressure because gravity aids the movement of blood toward the heart. The internal jugular vein in this situation is normally partially collapsed. When blood "backs up" in the veins due to cardiac disease the internal jugular vein expands and pulsations in the vein are visible on the surface of the neck. The visible pulsations begin at the base of the neck and progress upward as CVP increases. A measurement of the height of the highest location on the neck where pulsations are visible provides a simple and non-invasive method for estimating CVP. The internal jugular vein is thereby used as a manometer to estimate CVP.
The above-mentioned procedure comprised the following steps: the head of the patient's bed is placed at an elevated angle, pulsations in the internal jugular vein are located, and the highest point at which these pulsations are visible is measured. Conventionally, the height of this point is measured from the sternal angle, also called the angle of Louis, which is a reference point on the sternum. The sternal angle is roughly 5 centimeters above the right atrium. The height measurement is taken by placing the base of a centimeter ruler on the sternal angle while the ruler is held in a vertical orientation. A tongue depressor or other straight object is then placed at a right angle with respect to the ruler and is used to locate the highest visible pulsations. A physician may also merely estimate the height visually. If the highest visible pulsations are more than a specified number of centimeters above the sternal angle then the CVP is considered elevated. A diagram indicating this method is shown in FIG. 5.
Several other methods or devices may be used to estimate CVP. Rockwell, in U.S. Pat. No. 3,413,970, discloses an integral, one piece arm which supports a slidable member. The slidable member is extended to a prescribed position under a patient to establish the vertical position of the patient's superior vena cava, after which a scale on the arm may be used to set a baseline for a manometer. The arm includes a level indicating means afixed thereto. The CVP measurement is invasive. Sackner, in U.S. Pat. Nos. 4,452,252, 4,456,015, 4,986,277, and 5,040,540, discloses several transducers which are wrapped around a patient's neck to measure changes in the cross-sectional area of the neck. Those changes are related to CVP. Allocca, in U.S. Pat. No. 4,204,547, discloses a method of monitoring intracranial pressure by occluding the jugular vein at a particular point and measuring the rate of change of blood pressure upstream of that point. Soviet Patent No. SU-452332-A discloses the measurement of venous pressure while the patient's inclined position is raised.
The need remains for an accurate and non-invasive method for estimating central venous pressure.